The present invention relates to a central symmetric Gamma voltage correction circuit, which is mainly used to the displaying circuit of a liquid-crystal display. A circuit formed by a plurality of resistors, varistors and amplifiers. This, the number of the correction voltages input externally is reduced, and amplifier required can also be reduced.
A Gamma voltage correction circuit is used to an active matrix liquid-crystal display. The main function thereof is to provide a digital coded signal converter. With respect the characteristic curve of a liquid-crystal display, the input image data is adjusted properly along a curve way. Through this conversion characteristic curve, the hue, gray level, contrast and color of the display can be adjusted.
With reference to FIGS. 1A to 1D, wherein FIG. 1A shows the relation of image data codes to the displaying property (T) of a liquid-crystal display, where T can be transmittance, hue, gray level, contrast, or color, etc. FIG. 1B shows the relation of the voltages in a general liquid-crystal display to the displaying property (T) of a liquid crystal display. FIG. 1C is a characteristic curve of image codes of liquid-crystal display relative to FIG. 1A. If it is desired to acquire the characteristic curve of FIG. 1C, an adjusting mechanism is necessary for compensating the change of the property of the display due to outer data to be input into the display. The adjusting mechanism is Gamma correction voltage. FIG. 1D shows a conversion curve of the data codes of Gamma voltage correction circuit relative to the voltages. In a TN(Twisted-Nematic) LCD, the characteristic curve of the transmittance of the liquid-crystal material to the voltage is a nonlinear curve. Therefore, in Gamma voltage circuit, the more the sampling points of the reference voltage, the smaller the approaching error of the characteristic curve can be obtained. In the trend of high resolution, for example, an 8-bit data driver for providing 256 gray levels, if it is desired to give an optimum adjustment to these 256 gray levels, the adjusting work is made through 256 reference voltage points which is provided externally. Furthermore, the adjusting work is performed one by one. However, the driving voltage of liquid-crystal material must be alternative voltage, and therefore, each of the positive and negative polarities needs 256 reference voltages. Totally, 512 external input reference voltages are necessary for adjustment, but it is impractical to make so many inputs of the reference voltage in one driving IC. In fact, it is seldom to make such a work. Therefore, in general, only a few reference voltages are provided externally, and then in the driving IC, by a voltage dividing way with a fixing ratio, the desired reference voltages without being provided externally are acquired by voltage dividing. However, these reference voltages from the resistor voltage dividing circuit must be confined by the externally provided reference voltages and the voltage dividing resistances. Further, the characteristic curve of the liquid-crystal display will be confined, namely, a larger error occurs as to approach the characteristic curve.
With reference to FIG. 2, a Gamma correction voltage with a fixed ratio resistor voltage dividing is illustrated. As shown in FIG. 2, in the driving of the general DC Gamma voltage, the data driver 3 generally needs a set of central symmetric Gamma correction voltage input. This central voltage is obtained from Vcom=(Vcc+VGND)/2. The input voltage (Vcc, VGND) passes through a resistor voltage dividing circuit 1 for voltage dividing so as to obtain a plurality of voltage dividing points. Then these points are transferred to the driving circuit 2 for gain-amplifying and then is transferred to a data driver 3 for identifying the correction voltages for driving the positive and negative polarities. FIG. 2 shows a way of voltage dividing by serial resistors to adjust a plurality of output voltage points. In this circuit, it is hard to properly adjust the levels of the voltages and to adjust the center voltages of the positive and negative polarities to be symmetric. Therefore, in this circuit structure, if any resistance is changed, other output voltages will be changed.
With reference to FIG. 3, a characteristic curve for the photoelectric effect for the voltage driving of general liquid-crystal displays. The relation of the driving voltage with respect to the displaying property of the display is illustrated. The Vcom, defined as common voltage, in the drawing is a center voltage of the characteristic curve. The value of the central voltage is determined from an external voltage. The characteristic curve is symmetric at two sides of the central voltage, and a positive polarity region and a negative polarity region are classified at two sides of the central voltage. These positive polarity region and negative polarity region are the sources of the positive polarity voltage and negative polarity voltage required by the liquid-crystal display.
Accordingly, the primary object of the present invention is to provide a central symmetric Gamma voltage correction circuit, by the present invention, the displaying property of liquid-crystal display may be improved.
Another object of the present invention is to provide a central symmetric Gamma voltage correction circuit, wherein a well adjustment way to the Gamma correction voltage can be acquired.
A further object of the present invention is to provide a central symmetric Gamma voltage correction circuit, wherein the Gamma correction voltage can be controlled by externally inputting voltage so as to realize a simpler and flexible control way.
Yet, an object of the present invention is to provide a central symmetric Gamma voltage correction circuit, wherein by reducing the number of the Gamma voltage circuit, the number of the components in the circuit is also reduced.
A still object of the present invention is to provide a central symmetric Gamma voltage correction circuit, wherein by reducing the number of the externally input correction voltage in the Gamma coefficient circuit, the number of pins for inputting data to the Gamma correction voltage can be reduced.
In order to achieve the aforesaid object, the present invention provides a central symmetric Gamma voltage correction circuit for improving the defects in the prior art. In a basic circuit, by a circuit formed by resistors, adjustable resistors and amplifiers, a voltage is externally input and the voltage is divided by the resistors, varistors and amplifiers. After the varistors are adjusted, two ends of the varistors will acquire a positive polarity voltage and a negative polarity voltage.
In a preferred embodiment that the present invention is connected to a data driver, if the number of the input correction voltages required by the data driver is 2N, then through the preferred design of the present invention, a half of the coefficients are remained to be connected to the data driver by the OP buffer of the driving circuit, while another half are output by the two ends of the varistors of the Gamma voltage correction circuit without needing to be connected to the OP buffer.
Through the design of the present invention, the number of the externally inputting Gamma correction voltages is reduced to a minimum value, while for the correction voltages not being input externally can be acquired by a voltage dividing circuit and varistors.
The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawing.